Conventional gas appliances and heating equipment, such as gas ranges, often use silicon carbide (SiC) hot surface ignitors or spark ignitors. The conventional SiC ignitor is designed to survive in the gas range environment. The SiC ignitor is normally placed in series with the gas valve. The gas valve is designed to open when the current supplied to it exceeds a certain value. The SiC ignitor has a carefully controlled resistance versus temperature characteristic such that: (1) when current is initially supplied to the ignitor and the ignitor is cold, it has a relatively high resistance that keeps the current low enough so the gas valve stays closed; and (2) when the ignitor heats up, the resistance drops so the current becomes sufficiently large to open the gas valve. When the current reaches this threshold point, the ignitor is hot enough to ignite the gas. This resistance versus temperature relationship serves as a "fail-safe" in that the ignitor must reach a certain temperature before the gas valve opens, thus avoiding the situation of gas flowing to an ignitor which is not hot enough to ignite the gas.
Conventional SiC gas range ignitors are produced by several commercial vendors, including Surface Igniter Co. of Chagrin Falls, Ohio and Saint-Gobain/Norton Co. of Milford, N.H. Some of the problems with these conventional ignitors are that they are porous, fragile, and expensive. In addition, the resistance versus temperature characteristics of these conventional SiC ignitors may alter or drift over time, thereby adversely affecting their reliability.
Ignitor materials which are more mechanically robust than SiC have also been developed. One such ignitor, the Mini-Ignitor.RTM., available from the Saint-Gobain/Norton Company of Milford, N.H., comprises a pressure sintered composite of aluminum nitride ("AlN"), molybdenum disilicide ("MoSi.sub.2 "), and silicon carbide ("SiC"), and is designed for 8 volt through 48 volt applications. However, the resistance versus temperature characteristics of the pressure sintered composite material is different from the resistance characteristics of conventional ignitor materials such as SiC. Generally, the pressure sintered composite material has a resistance which increases with temperature (e.g., a metallic resistance characteristic). Accordingly, pressure sintered composite ignitors are generally not compatible with existing conventional ignition systems which rely on a resistance fail safe region.
Thus, there is a need for a reliable ignition system which does not rely on a resistance fail safe region and which is not susceptible to performance degradation due to temperature drifts.